For starters, because Zoom's decision to go with either of those companies as a cloud supplier isn't going to have any impact on their competition as a conferencing service provider. It's not like Microsoft is going to go broke tomorrow and cancel teams if Zoom goes to Oracle.

More fundamentally because business is cooperative and not like a war, but like a game with many repetitions. So working with your competitors can be mutually beneficial. Business is not a zero-sum game, it's not like two villages in the walking dead going at each other.